1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the zoom lens, and more specifically, it is suitable for a digital still camera, a video camera, a TV camera, a surveillance camera, a silver-halide film camera, and the like.
2. Description of the Related Art
In recent years, an image taking optical system used in an image pickup apparatus such as a digital camera or a video camera has been required to be a compact zoom lens having a high zoom ratio, a wide angle of view, and a high resolution. An image taking optical system used in a digital still camera has been required to record not only still images but also movies and to be a high-optical-performance yet compact lens system.
A positive-lead type zoom lens having a lens unit of positive refractive power disposed closest to the object is known as a compact and high-zoom-ratio zoom lens. As a positive-lead type zoom lens, there is known a four-unit zoom lens including, in order from the object side, first to fourth lens units of positive, negative, positive, and positive refractive powers, wherein each lens unit moves during zooming, and focusing is performed by the fourth lens unit (see Japanese Patent Laid-Open No. 2007-47538 and U.S. Pat. No. 7,944,622).
As a positive-lead type zoom lens, there is known a five-unit zoom lens system including, in order from the object side, first to fifth lens units of positive, negative, positive, negative, and positive refractive powers (U.S. Pat. No. 7,304,805).
It is relatively easy to increase the zoom ratio of a positive-lead type zoom lens while downsizing the entire system thereof. In order to downsize the entire system of a positive-lead type zoom lens while ensuring a predetermined zoom ratio, it is recommended to reduce the number of lenses while increasing the refractive powers of lens units constituting the zoom lens. However, in such a zoom lens, the lens thicknesses increase with increase in refractive power of lens surfaces. As a result, the downsizing of the entire lens system becomes insufficient, and aberrations often occur.
In an image pickup apparatus whose lens units are retracted when not in use, errors such as inclination of lenses and lens units are inevitably large due to the mechanical structure. If the lenses and the lens units are sensitive, the optical performance degrades and image shake occurs during zooming.
For this reason, it is necessary to minimize the sensitivities of the lens and the lens units to obtain high optical performance. In the above-described positive-lead-type four-unit or five-unit zoom lens system, it is important to appropriately set each component of the zoom lens to obtain high optical performance while downsizing the entire system and increasing the zoom ratio. For example, it is important to appropriately set the zoom type (the number of lens units and the refractive power of each lens unit), the movement locus of each lens unit during zooming, and the share of zoom ratio of each lens unit.
In particular, it is important to appropriately set the lens configuration and the refractive power of the first lens unit, and the share of zoom ratio of the second lens unit for magnification variation.
If these are not appropriate, the size of the entire system increases when the zoom ratio is increased, the variation in aberrations during zooming increases, and it is very difficult to obtain high optical performance throughout the zoom range and throughout the viewing area.